The present invention relates to a suction air measuring device for use in an electronic controlled fuel injection device of an automobile engine.
A method has been known in, for example, Japanese Utility Model Application Laid-Open No. 41665/1979, in which a vortex generating member is provided in a conduit and a flow rate of fluid passing therethrough is measured on the basis of the occurrence frequency of Karman vortex generated downstream of the vortex generating member. This method utilizes the fact that an ultrasonic signal wave transmitted or received through the conduit is phase-modulated by Karman vortex generated in the fluid. By demodulating the signal, the vortex signal is detected as an analog electric signal.
The analog electric signal is usually binarized by comparing it with a predetermined voltage by a voltage comparator and outputted as a Karman vortex pulse signal having a frequency equal to the occurence frequency of the vortex.
A use of such Karman vortex flow rate sensor as a suction air sensor of an automobile engine provides a problem that in, when a throttle valve of the engine is opened substantially, a pulsation of air due to a suction operation of the engine is produced in a suction conduit and an output level of the electrical signal is changed largely according to the pulsation of suction air.
When such an output signal is binarized according to the known method, a pulse drop problem occurs. In order to solve this problem, Japanese Utility Model Application Laid-Open No. 72119/1982 proposes the removal of the fluctuation of the analog electric signal due to pulsation of suction air by passing it through a differentiation circuit. In this solution, however, the signal level is lowered when a frequency thereof is low.
In order to remove the latter problem, in 72119/1982, an input characteristic of the differentiation circuit is changed according to an output of the pulse circuit, i.e., according to the pulse frequency of Karman vortex generated by the measuring device itself. Therefore, if such pulse drop once occurs, the pulse drop situation continues.